Controlled-release fertilizer coated with polyurethane is prepared from the direct reaction of the reactants of polyol and isocyanate etc. on the surface of the fertilizer particles. Beside the raw materials formulation of the polyurethane reaction, it will also be affected by controlling technology of the process including spray coating for film formation and particle fluidization etc.
At present, mainly the castor oil polyol and isocyanate are used as the reactants. According to the reaction stoichiometry, the reactants will be added directly onto the surface of the circularly rolling fertilizer particles for reaction and film forming to prepare the controlled-release fertilizer. Early in 1986 William P. Moore et al. in the United States had used the rotating drum to make MDI and polyester polyols react on the surface of the sulfur coated urea to prepare the polyurethane film. The invention uses laboratory rotating drum coating device with a diameter of 24 inches and a length of 10 inches for coating operation, wherein, in the rotating drum, 15 shoveling plates are disposed to assist particles rolling, the rotating speed of the drum is 30 rpm, temperature is 110° C., and 4 kg sulfur coated urea is added for once. During coating, 1.0% isocyanate is firstly sprayed into the drum, after 2 minutes of rolling, 1.5% polyol is added and then rolling is allowed for two minutes, which is repeated several times. Triethanolamine or melamine is used in reaction as catalyst. In the study the influence of catalyst, reaction temperature, NCO/OH mixing ratio and other factors on coating effect had been discussed (U.S. Pat. No. 4,711,659). The patent had used the polyurethane reaction to prepare the controlled-release fertilizer firstly, but in the study the influence of reaction and coating conditions on the nutrition fertilization performance of the polymer coated fertilizer had not been analyzed thoroughly. Furthermore, the active speed of the reaction of the selected raw materials is relatively slow, the reaction temperature is relatively high (100˜150° C.), and water resistance of the polyesters is relatively poor, thus its 24 hour water dissolution rate at 100° F. is 7.1%, the controlled releasing performance is not satisfactory, and there was no report of industrial production. The controlled-release fertilizer coated with wearable resin, which is applied for by William P. Moore in 1987, has employed similar ideas, wherein HO— and NH2— are employed as nucleophilic reagent to react with NCO—, which had changed the type of reactants, but the reaction conditions had not been improved, thereby the fertilizer performance of the prepared controlled-release fertilizer had not been changed (U.S. Pat. No. 4,804,403).
In 1994 Alice P. Hudson used the round disc as the coating device, used the reaction of isocyanate (DOW PAPI 94) and hydrogenated castor oil (Union Camp Cenwax G) to prepare controlled-release fertilizer coated with the polyurethane and used paraffin wax (Gulftene C30+, alpha olefin) accounted for 10-50% of the total weight of the reactants to modify the urea surface; the reaction temperature is about 100° C.; hydrogenated castor oil was intermittently added; isocyanate was continuously added; the reactants account for 1˜10% of total weight of the coated fertilizer (mainly 1-3%); the effect of NCO/HO disposition and used wax amount on the releasing performance is compared with emphasis; coated controlled-release fertilizer with about 20% releasing for 7 days can be obtained (U.S. Pat. No. 5,538,531). In 1999 Yasuhiro Hirano et al. employed electriferous heater with a diameter of 520 mm and temperature-controlled disk, when the rotational speed reached 20˜30 rpm and temperature was 75° C., 2 kg fertilizer particles was placed in. Firstly, 4.1 g MDI (Sumidule 44V10, Bayer) and 5.3 g castor oil (Sumifen™, Bayer), and 0.1 g 2,4,6-tris (dimethylaminoethyl) phenol were used as catalyst, which were sprayed onto the surface of the fertilizer particles for reaction for 3 minutes. This is repeated 16 times and the total coating rate can reach to 8%. Afterwards paraffin wax is added and cooled and solidified into film; the releasing period can be up to about six months (U.S. Pat. No. 6,231,633). In China there are also patents in aspect to the fertilizer coated with polyurethane, wherein the rotating drum was mainly used, reaction materials were added intermittently, and the process of reaction-solidifying is repeated several times. For example, in the rotating drum, materials A, B was dropwised through the nozzle onto the surface of fertilizer particles within the rotating drum to form a coating layer. After reaction and solidifying, then the coating was repeated 2˜10 times (CN 101648837A). In addition, the Chinese Patent applied by Ivory Company Limited has introduced the method for producing the fertilizer coated with polyurethane by rotating drum. Its specific operation is: adding 1 kg urea into the rotating drum and heating up to 75° C.; then adding castor oil containing 18% paraffin wax, then adding isocyanate, rotating for 6 minutes, and then repeating the operation steps three times; afterwards removing the heat source, cooling down for 12 minutes until about 30° C., and moving out the test samples. The invention also stressed that adjusting the drum with different rotation rate at different stages of the operation had great impact on product performance.
At present, the method widely used is the method of single dropping for intervals, i.e. reactants are dropped and atomized onto the surface of the fertilizer particles according to the sequential order and the method of adding for intervals. In this way the polyol first added will collide and contact with each other in particle fluidization, to form uniform coating layers. Afterwards, isocyanate is added for reaction to prepare polyurethane film.
In addition, fluidized bed as the coating apparatus generally comprises of a gas distribution chamber, a gas distribution plate, fluidized chamber, enlargement segment, inner components and feed inlet and discharge etc. The structure and inner components vary much depending on the specific process requirement. At present, the devices used in the production of coated fertilizers are mainly Wurster fluidized bed, spouted bed, rotating drum fluidized bed and column style fluidized bed. These devices also have some drawbacks as coating equipment.
Currently polyols used in polyurethane coating reaction are mainly polyols of animal and vegetable fats and partially polyester polyol. Isocyanates used in the reaction are mainly toluene-2,4-diisocyanate (TDI) and diphenylmethane diisocyanate (MDI).
In summary, in the current technological method used in the production of polyurethane coated fertilizers, the reaction material is added intermittently, the operation is relatively complicated and difficult to be carried out automatically and continuously, thus it has some drawbacks:
1. The reaction material is mostly added alone and sequentially. After each reaction monomer is added, a period of time is usually needed to allow its even distribution, thus leading to the disadvantage that reaction material are distributed unevenly and the reaction products are difficult to control;
2. the reaction material is added intermittently and will subject to three phases of uniformly coating, reaction and solidifying for film forming. Especially in the phase of solidifying, viscosity of the polymer will increase in a sudden, therefore the phenomenon of adhesion clusters of the particles is easy to occur, thus interrupting the operation. However strongly stirring the adhesion clusters is easy to destroy the film, thereby the controlled releasing performance is very unstable;
3. In the prior continuous fluidized bed or spouted bed, a number of bed bodies are simply series connected. When the material enters the next bed body from the previous bed bodies, special components plus manual control are needed, which belongs to semi continuous operation. The fluidization phenomenon of the particles is not made full use to make them automatically and gradually flow from one end to the other;
4. Most of the current coating technology are developed from the castoryl polyurethane reaction, therefore the adjustment range of structural performance of the polyurethane products is very limited and controlled releasing performance of the coated fertilizer cannot be improved through controlling the reactants. The isocyanates used in the current patent are mainly TDI and MDI. The former one has great toxicity. The latter one is solid at normal temperature and its chemical properties are easy to change, thus its use is not convenient. However, as the reaction speed is taken into account, MDI is mainly used, which makes MDI addition process very complex.